1. Field of the Invention
The invention is in the field of hydraulic energy dissipation and flow training in vertical conduits.
2. The Prior Art
Many situations arise in hydrualic engineering (and other branches of fluids engineering) in which large flows of water or other liquids must be conveyed from a higher to a lower elevation. Some examples of such situations:
outlet structures and spillways in dams; PA1 bypasses around hydraulic turbines; PA1 bypass and overflow structures in cooling towers; and PA1 vertical conveyances for conducting stormwater surface runoff to underground tunnels, as in the TARP system of the city of Chicago.
In these situations the water falls through vertical conduits called dropshafts. The potential energy of the water at the higher elevation is converted into kinetic energy as the flow falls down the dropshaft, and this energy must be dissipated at the bottom of the dropshaft in such a way that it does not damage the structure. The falling water also entrains large quantities of air, which in many situations must be separated from the water and somehow vented to the surface before the water enters the conduit which conveys it away from the dropshaft. Otherwise the entrained air will coalesce in the conduit into large bubbles, which, being under considerable pressure at the depth of the conduit, can force their way up another dropshaft or a venting structure with energy sufficient to cause extensive damage.
Over the years two principal types of dropshafts have evolved. In one type, a structure at the entrance to the dropshaft conveys water to the top of the dropshaft, where it bends sharply downward in a vertical plane to direct the water downward. The water falls freely down the dropshaft. Collectively, such designs are called plunge-flow dropshafts. An example of a plunge-flow dropshaft is the simple case of an inflow conduit with a short radius elbow in a vertical plane. Plunge-flow dropshafts have a drawback: Water cascading down the dropshaft entrains large volumes of air. Large and often complex subterranean chambers are required to collect the entrained air and to vent it to the surface through special vents provided for the purpose. Moreover the kinetic energy of the falling water must be dissipated safely in a subterranean structure called a "plunge pool" or "energy dissipator".
A second design for dropshafts is the vortex type. In a vortex-type dropshaft, water is delivered into the top of the dropshaft tangentially and is directed slightly downward, so that the water swirls as it falls down the dropshaft. Inlets to vortex type dropshafts take several forms: spiral inlets, scroll inlets, and tangential inlets, among others. Centrifugal force resulting from the rotation of the water holds the flow against the dropshaft wall, forming a central vent through which entrained air can escape to the surface. The flow remains in contact with the dropshaft wall, and much of the energy dissipation occurs in wall friction. However, the high velocity of the free-falling spiral flow down the dropshaft, and its energy into the flooded pool at the bottom of the dropshaft, entrains large amounts of air. Therefore, a deaeration chamber must be installed at the bottom of vortex-type dropstructures that discharge into closed conduits, and the deaeration chamber must be vented to the surface.
The subterranean energy-dissipation and deaeration structures at the base of a dropshaft constitute a major fraction of the cost of the structure. Moreover, if they do not function as intended, there may result extensive damage to the dropshaft and the structure appurtenant to it. There exists therefore a need for a dropshaft design which minimizes the amount of energy which must be dissipated at the bottom of the dropshaft, which minimizes entrainment of air in the falling water, and which functions reliably over a wide range of discharges.